I've organized this according to the level of content at which you're most likely to encounter the mounts, from classic Azeroth dungeons on up.
The evil blue horsey was once famous as the rarest dropped mount in the game, supposedly dropping at a nightmarish .01% rate. Or so the playerbase estimated; the drop rate was so incredibly low that nobody was really sure just how low it was. As another kick to the teeth, until patch 1.6.0, you only ever got one chance to kill Baron Rivendare per run. If you wiped on him, which was a distinct possibility at the time due to his constant damage aura and numerous adds, he'd despawn.
Someone trotting around on the Deathcharger back in the day is the sort of person who should be actively encouraged to buy lottery tickets and play the ponies, 'cause ... damn.
Where you'll find it Baron Rivendare -- once the end boss of the undead side of Stratholme and the original owner of the Deathcharger -- took a little road trip in Wrath of the Lich King to the death knight starting experience, and afterwards set up shop in Naxxramas on the Four Horsemen encounter. He's left his evil horsey and his loot table in the care of a relative, so nowadays you'll find the Deathcharger (sometimes) on Lord Aurius Rivendare's corpse.
Drop rate These days, a far more forgiving 0.8-1%.
Also the subject of Deathcharger's Reins, a feat of strength
While technically a re-skin of the troll raptor mounts, there's no denying that the Raven Lord mount is irresistibly badass. It was once available only to druids (or anyone in party with one) as part of their Swift Flight Form quest series. After you'd completed the quests and gotten your equally badass epic flight form, you could pick up an Essence-Infused Moonstone for future resummons. This led to a number of players "hiring" druids for runs and/or summons in Sethekk Halls for a chance at the mount. Fortunately, anyone can get it these days.
Where you'll find it It's dropped by Anzu, a special boss that spawns only in heroic Sethekk Halls. You'll have to get to honored with the Lower City faction of The Burning Crusade in order to purchase the Auchenai Key granting you entry to the heroic version of the dungeon, but that's pretty easy. Questing in Terokkar Forest or a few runs of normal Auchenai Crypts/Sethekk Halls/Shadow Labyrinth should get you to honored quickly.
Drop rate 1%
Also the subject of Reins of the Raven Lord, a feat of strength
Swift White Hawkstrider
What I remember most about this mount is its being the only hawkstrider in the game that tauren could ride at the time ... I think? I'm not sure that tauren could even ride the PvP hawkstrider before Blizzard finally caved and made every mount available to each race (by the way, that's the first article that Sacco ever wrote for WoW Insider). This was the first rare mount I ever won in a roll-off, and every time I popped my female tauren main onto it, I started laughing. There is no way to make a giant cow perched on an equally giant chicken look dignified.
Where you'll find it Dropped by Kael'thas "Tempest Keep was merely a setback!" Sunstrider in heroic Magisters' Terrace. MgT (we tried valiantly to have it nicknamed MrT, but no dice) was arguably the most dangerous heroic of The Burning Crusade era, but these days most classes can solo it without too much difficulty.
Drop rate 1%
Also the subject of Swift White Hawkstrider, a feat of strength
The Bronze Drake was Blizzard's experiment with putting a Zul'Aman-esque timed run mechanic in 5-man content. Actually, scratch that; you can argue that the 45-minute Baron run in Strat was really the granddaddy of later timed runs, but all you really got from that was the satisfaction of saving some hapless NPC's life. (Question: What happened to Ysida Harmon in Cataclysm, anyway?) Needless to say, that is nowhere near as satisfying as acquiring another mount.
Where you'll find it Dropped by the Infinite Corruptor, a mob that spawns in the heroic version of the Culling of Stratholme. You've got 25 minutes from the true start of the dungeon (i.e., once all players have zoned into the city proper; you should see a countdown at the top of your screen) to reach him. At level 85, this should be a cakewalk, and even for well-geared level 80s, it was almost never an issue. However, it was a pretty reasonable challenge in the first few months of Wrath of the Lich King.
Drop rate 100%. As long as you can kill the Corruptor before time runs out (and you do have to kill him; if you're fighting him when the clock hits zero, he'll vanish), you've got yourself a drake.
For a lot of players, this was their first introduction to the very cool new proto-drakes of Wrath. Every other proto-drake at that point was either an incredibly rare spawn, an equally rare chance at the end of a long reputation grind, the subject of a lengthy set of achievements in 5-mans, 10-mans, or 25-mans, or the reward for a year's worth of effort.
Where you'll find it Dropped by Skadi the Ruthless in heroic Utgarde Pinnacle. Skadi was one of the tougher bosses in the early part of the expansion, though I say that as a bear tank once saddled with a non-360 degree, must-have-target Swipe. These days a few classes can solo heroic UP without difficulty.
Drop rate 1%
I'm cheating here a little by including the Blue Drake, which pulled double duty in Wrath as both a raid drop and -- much later -- an incentive to keep players from running away screaming from Oculus. This wasn't a giant issue before December 2009, because the people who had to do Oculus for whatever reason usually made a group specifically for that purpose, and most others simply avoided it. Once the dungeon finder debuted in patch 3.3, lots of players suddenly found themselves "randomed" into the most widely-hated instance in the game and responded by getting the hell out of Dodge. Ironically, the number of players dropping the dungeon made it all the more likely that other people entering the queue would land the place, because the dungeon finder prioritizes any run where people are already in the instance. Players coming into an existing Oculus run usually responded by getting the hell out of Dodge. I was once in an Oculus where no fewer than 7 players dropped before we'd finished pulling the opening hall.
Nightmare. So Blizzard decided to sweeten the deal with extra badges, gems, and a chance at the Blue Drake. Players responded by thinking for a few seconds before getting the hell out of Dodge. Nah, most stayed. Apparently, players' price for suffering through a heroic they don't like is a very small chance at a mount, so Blizzard is perhaps correct for thinking that history will repeat itself. We'll find out soon enough.
Where you'll find it You won't find it there nowadays, but it was once a rare drop from something called the Cache of the Ley-Guardian, a goody bag awarded to players who successfully completed heroic Oculus. Today it's gone back to being a random drop off the 10-man version of Malygos.
Drop rate 1%
Vitreous Stone Drake
I've privately wondered if this thing is the latter-day version of the Blue Drake, because Stonecore is arguably Cataclysm's most unforgiving dungeon. Then again, if that were true, it'd be somewhat counterproductive to have the dungeon's second (and easiest) boss dropping it, so maybe not.
Where you'll find it Dropped by Slabhide in both the normal and heroic versions of Stonecore.
Drop rate 0.8-1%. It almost invariably goes to the run's worst player.
Drake of the North Wind
One of the very cool-looking new drake models, but you'll have to suffer through one of Cataclysm's more annoying bosses to have a shot at it.
Where you'll find it Dropped by Altairus in heroic Vortex Pinnacle. Fortunately for us, this boyo's come in for a few recent nerfs. I tend to agree with Fox here that the nerf was well-deserved, not least because Lightning Blast was nothing more than a big ol' "Screw you" to someone who'd been tossed from tornado to tornado before finally getting tossed off the platform.
Drop rate 0.8-1%
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